Not eating, blood tests normal.

Hello, Dr. Marie. I'm actually asking this question for my parents about their cat, Josie. She's going to be 13 years old in a few days, and she has stopped eating.
She's always been pretty fussy about eating, but lately she's just stopped. They've tried several different types of food, baby food, tuna, salmon, crab, you name it. Currently, they are force-feeding her with a syringe. They took her to a vet about a week and a half ago (she's never had a regular vet before. Since, she's been back about 4 times), who did x-rays and bloodwork. The x-ray initially showed a shadow near her esophagus, but the bloodwork came up fine. The vet sent her to an oncologist to get an ultrasound done to see what the shadow was, but when they did the ultrasound, the shadow was gone. The ultrasound also came back clear (it showed a tiny amount of food in her stomach that makes me wonder if that WAS the shadow). The oncologist, however, noted that a number of Josie's teeth were loose. She went back to the vet, who pulled about 8 of her teeth. We initially thought that this would be the answer to the problem, but it's been close to a week, and she still won't eat on her own.
As a bit of background, Josie is the littermate of our cat Emily. Emily passed away in October 2012 after dealing with cancer on her lung and undergoing chemotherapy; she was diagnosed with the cancer in June 2012. Previous to that, Emily had suffered a bout of Irritable Bowel Disease around Christmas 2011. She, too, wasn't eating well and wasn't pooping, but she was also vomiting; Josie isn't vomiting, she just looks at food and walks away.
Josie is drinking quite a bit, according to my parents, so she is staying hydrated. And they are force-feeding her with the syringe. Other than that, they say she's acting the same as she always has. She walks around the same, her energy level is the same, she meows the same amount. It's strange!
I'm concerned for a number of reasons, but one thing that got me was how the vet had missed the loose teeth! Every vet I've ever taken my pets to look into the cats mouth to look at their teeth. According to my dad, the vet never looked into her mouth, and the first they knew about the loose teeth was from the oncologist. Based on that alone, I said they should get a second opinion. :)
Any ideas about what might be going on? I've told them how they need to keep force-feeding her to keep off fatty liver disease, and they are continuing to do that. But it's just a puzzle as to why nothing shows up on any blood work, or why the x-ray showed a shadow near her esophagus and the ultrasound later showed nothing. The oncologist also checked the ultrasound for any enlarging of her liver or any organs; nothing.
I'm scared of cancer, especially how we lost Emily. We know that they have a bit of Siamese in their bloodline that might account for the occasional anorexia/bulemia spells (Emily used to eat a bunch of food once in a while and immediately barf it back up), but Josie's never done this before. Fussy, yes. Off food completely, no.
As you can guess, my parents are at their wits end. Any words of advice? Should they at least go get a second opinion? I've tried to find a few causes that might cause Josie to do this: more abcessed teeth or gum disease, inflamation of the pancreas, kidney infection, cancer. But I would have thought SOMETHING would show on the ultrasound or the bloodwork for any of these. HELP!!!
Thank you so much for your patience with this letter. Anything you can offer would be wonderful. I'll try to answer any other questions as soon as I can. Thanks again. :)
Liza